Gaara, using his ability to manipulate sand, loosens the ground below his enemy and catches them in a maelstrom of sand. The swallowed enemy then sinks up to two hundred metres underground, while completely unable to move a muscle. After that, the sand glued onto their limbs applies pressure on their whole body, so they can't even twitch a finger. The pressure resulting from being buried at such an extreme depth would surely kill any normal human, if not from suffocation, then from the fact that all their bones would break.